1. Field of the Invention
This invention relates to accelerometers and more particularly to a class of accelerometers that use differential Eddy current sensing to provide improved sensitivity at lower cost and with higher reliability.
2. Description of the Related Art
The basic, open-loop accelerometer consists of a proof mass attached to a spring. The mass is constrained to move only in-line with the spring. Acceleration causes deflection of the mass. The displacement of the mass is measured. The acceleration is derived from the values of displacement, mass, and the spring constant. The system must also be damped to avoid oscillation. A closed-loop accelerometer typically achieves higher performance by using a feedback loop to cancel the deflection, thus keeping the mass nearly stationary. Whenever the mass deflects, the feedback loop causes an electric coil to apply an equally negative force on the mass, canceling the motion. Acceleration is derived from the amount of negative force applied. Because the mass barely moves, the non-linearities of the spring and damping system are greatly reduced. In addition, this accelerometer provides for increased bandwidth past the natural frequency of the sensing element. (Excerpted from Wikipedia “Inertial Navigation System” and “Accelerometer”).
Conceptually, an accelerometer behaves as a damped mass on as spring. When the accelerometer experiences acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the accelerometer body. The displacement is then measured to give the acceleration.
In commercial devices, piezoelectric, piezoresistive and capacitive components are commonly used to convert the mechanical motion into an electrical signal that is proportion to the displacement of the mass. Piezoelectric accelerometers rely on piezoceramics (e.g. lead zirconate titanate) or single crystals (e.g. quartz, tourmaline). They are unmatched in terms of their upper frequency range, low packaged weight and high temperature range. Piezoresistive accelerometers are preferred in high shock applications. Capacitive accelerometers typically use a silicon micro-machined sensing element in which the proof mass forms one side of the capacitive sense element. Their performance is superior in the low frequency range and they can be operated in servo mode to achieve high stability and linearity.
Modern accelerometers are often small micro electro-mechanical systems (MEMS), and are indeed the simplest MEMS devices possible, consisting of little more than a cantilever beam with a proof mass. Damping results from the residual gas sealed in the device. As long as the Q-factor is not too low, damping does not result in a lower sensitivity. Under the influence of external accelerations the proof mass deflects from its neutral position. This deflection is measured in an analog or digital manner. Most commonly, the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple, reliable, and inexpensive.
The performance of an accelerometer is primarily a combination of its bias stability and scale factor error. Bias stability is the acceleration measured by the device if the actual acceleration is zero. If the device is not accelerating due to imperfections of the device and electronics the readout will be nonzero. The scale factor error reflects the error as proportional to the actual acceleration. If for example the device is accelerating at 1 g (32 meters/sec/sec) and the device reads out 1.1 g, the scale factor error is 10%.
As shown in FIG. 1, the open-loop accelerometer with capacitive or piezoelectric sensing and the MEMs accelerometer exhibit bias stabilities typically much greater than 10 micro-g's and scale factor errors greater than 10 ppm. Although this level of performance is adequate for many commercial applications it is not sufficient for “strategic grade” military navigation systems that require bias stability less than 10 micro-g's and scale factor error less than 10 ppm. Even when provided with servo control in a closed-loop configuration the performance does not satisfy strategic grade navigation requirements. A completely different approach to precision navigation is to provide the system with a GPS receiver and the use the network of GPS satellites to measure the position of the system. This approach enables navigation position accuracy in the 1 meter class. Without the GPS enhancement, this equates to accelerometer bias stability better than 0.1 micro-g's and scale factor error of less than 0.5 ppm. However, the GPS approach is inherently dependent on outside sources of information (i.e. the GPS satellites) and thus is not considered to be an option for strategic military systems that might have to operate in GPS denied environments or under conditions in which the GPS satellite network may be degraded.
Currently, strategic grade military systems use a Pendulous Integrating Gyroscopic Accelerometer (PIGA) in which a pendulous mass is free to pivot by being mourned on a hearing (Excerpted from Wikipedia “PIGA accelerometer”). A spinning gyroscope is attached such that it would restrain the pendulum against “falling” in the direction of acceleration. The pendulous mass and its attached gyroscope are themselves mounted on a pedestal that can be rotated by an electric torque motor. The rotational axis of this pedestal is mutually orthogonal to the spin axis of the gyroscope as well as the axis that the pendulum is free to move in. The axis of rotation of this pedestal is also in the direction of the measured acceleration. The position of the pendulum is sensed by precision electrical contacts or by optical or electromagnetic means. Should acceleration displace the pendulum arm from its null position the sensing mechanism will operate the torque motor and rotate the pedestal such that the property of gyroscopic precession restores the pendulum to its null position. The rate of rotation of the pedestal gives the acceleration while the total number of rotations of the shaft gives the speed, hence the term integrating in the PIGA acronym.
PIGA class accelerometers provide strategic grade performance without requiring any outside sources of information. However, PIGA class accelerometers are expensive to build and maintain. The number of moving parts reduces reliability and requires periodic maintenance and recalibration.